1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for examining patients. The present invention is particularly directed to a nuclear magnetic resonance apparatus as well as to a method for the operation thereof of the type wherein, respective images based on spin collectives exhibiting different chemical shifts are obtained.
2. Description of the Prior Art
Magnetic resonance tomography is a tomographic method for medical diagnostics that is distinguished by a high contrast resolution capability. Due to the excellent presentation of soft tissue, nuclear magnetic resonance tomography has developed into a method that is often superior to x-ray computed tomography. Magnetic resonance tomography is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality with measuring times on the order of magnitude of minutes.
In the presentation of the tissue of a patient, however, artifacts that arise from the influence of the chemical shift occur at the boundary layers between fat and water. Chemical shift is the phenomenon that the resonant frequency shifts slightly, proportional to the field strength, dependent on the type of chemical bond in which the atom containing nucleus participates. Due to its concentration in the human body, it is namely hydrogen nuclei of free water and of fat that contribute to the image. Their relative resonant frequency difference amounts to approximately 3 ppm (parts per million). The frequency difference leads to a relative shift of the images of the two spin type in the direction of the gradient that is active during the data acquisitions (read gradient or frequency coding gradient). The extent of the shift is dependent on the readout bandwidth per pixel, and thus on the field of view (FOV), and on the matrix size, among other things.
The original publication of W. T. Dixon, “Simple Proton Spectroscopic Imaging”, which appeared in 1984 in Radiology, Volume 153, pages 189-194, presented a method that achieves a separation of fat images and water images with two echos (gradient or spin echos). Dixon's idea for separating the image information of fat and water on the basis of the chemical shift shall only be discussed herein to the extent necessary for an understanding of the inventive method and algorithm set forth below.
The basis of the separation of the fat signal and the water signal according to the Dixon method is the chemical shift between fat protons and water protons. As mentioned above, this leads to different precession frequencies of the respective proton types. In a symmetrical spin echo experiment, phases that evolved due to chemical shift (or due to magnetic field inhomogeneities as well) are re-focused. Fat and water magnetization ideally are parallel to one another at the point in time of the echo. This condition can then be intentionally changed precisely such that fat and water magnetization reside perpendicular or anti-parallel relative to one another at the point-in-time of the echo. This is achieved, for example, by shifting the refocusing pulse by a time ΔT=π/2Δω (anti-parallel), or by ΔT=π/4Δω (perpendicular relative to one another) (Δω is the difference of the radian frequencies of fat and water protons due to the chemical shift). Of course, the relative magnetization orientations can also be generated with a suitable gradient echo sequence. When these three situations are then realized, i.e. when images are acquired with the corresponding phase evolution times, then the spin types can be locally prepared with phase relationships 0°, 90°, 180°. When the corresponding Fourier transformations are performed, three complex images are obtained that are composed of the spatially-dependent proton density of water (W(({right arrow over (r)})) and fat (F({right arrow over (r)}))):S0=(W({right arrow over (r)})+F({right arrow over (r)}))exp(iΦ0({right arrow over (r)})) S1=(W({right arrow over (r)})+iF({right arrow over (r)}))exp(iΦ0({right arrow over (r)})+Φ({right arrow over (r)})/2)) S2=(W({right arrow over (r)})−F({right arrow over (r)}))exp(i(Φ0({right arrow over (r)})+Φ({right arrow over (r)}))) S0 is the magnetization when water and fat are in-phase, i.e. exhibit an angle of 0° relative to one another. S1 is the magnetization at 90°, S2 the magnetization at 180°. Φ0 denotes the system-conditioned, spatially-dependent phase that has already built up in the first image due to B1 inhomogeneities of the excitation coils or due to the signal processing time in the spectrometer.
In the original Dixon method, only the images of parallel as well as anti-parallel alignment of the spin types relative to one another were employed, without taking field inhomogeneities into consideration. Algorithms for phase unwrapping of interconnected pixel regions (phase unwrapping) were developed in following publications by means of which field inhomogeneities could in fact be taken into consideration for the first time. These algorithms, however, were susceptible to error due to a region growth over regions having low signal-to-noise ratio. Phase unwrapping algorithms wherein the phase information were extrapolated over image regions without signal information proved equally susceptible to error. A good approach is represented by methods that (as above) initially enabled a pixel-by-pixel allocation of fat and water on the basis of parallel, anti-parallel and perpendicularly residing magnetization vectors and allowed a subsequent correction on the basis of local region growth. However, these methods have the disadvantage that the information available is not maximally utilized.